Certain clinical and cardiac morphologic findings are described in 22 patients, aged 45 to 80 years (mean 64) (15 men [68%]), in whom rupture of a papillary muscle occurred during acute myocardial infarction. In most, the acute myocardial infarction associated with papillary muscle rupture was their first coronary event (only 18% had myocardial scars consistent with prior infarction and 29% had angina pectoris). The posteromedial papillary muscle, presumably because of its more tenuous blood supply, ruptured almost 3 times more frequently than the anterolateral one (73% and 27%, respectively). Quantitative examination of the amounts of narrowing by atherosclerotic plaque in each of the 4 major epicardial coronary arteries (right, left main, left anterior descending and left circumflex) disclosed less narrowing in the rupture patients than in patients with fatal acute myocardial infarction unassociated with rupture. Of the 519 five-mm sections of coronary artery examined (11 patients), only 68 (13%) were narrowed greater than 75% in cross-sectional area compared to 34% of 1403 sections from 27 patients with fatal myocardial infarction without rupture.